Radio frequency signal processors, such as channelizers for analyzing radar receivers have typically employed the use of electro-optic components, such as Bragg cells, through which an acoustically modulated input optical (laser) beam is controllably modified (deflected) to become incident upon an array of photodetectors, the outputs of which define the frequency characteristics of the signal of interest. In addition, bulk wave signal processors that are effectively exclusively acoustic, and avoid the physical complexity shortcomings of acousto-optic devices, using focussed acoustic waves and operating at relatively low frequencies (on the order of several MHz), have been proposed for radio wave signal processing.
One example of such a focussed acoustic wave (FAW) signal processor, specifically an I.F. channelizer, is described in the U.S. Pat. No. 4,692,722 to Reichel. As detailed in the patent, a beam steering array of electro-acoustic elements, to which a signal to be analyzed is applied, is located on a first surface portion of bulk material into which the acoustic wave is launched by the array. Spaced apart from the beam steering array on a second surface portion of the bulk is an array of receiver transducers. The acoustic beam, the launch direction of which into the bulk is governed by the frequency of the input signal, is focussed by the bulk onto the receiver array for analysis.
Now, although the patented scheme is intended to obviate hardware and opto-electric component sensitivity limitations of conventional acousto optic signal processors, its useful frequency range is limited to a bandwidth considerably less than that to which acousto-optic signal processors are applied, so that one cannot simply make an acoustic processor for acousto-optic processor substitution and expect success. As noted above, the patented acoustic processing system is described as being operational at I.F. frequencies (the example given referencing a typical frequency range of 1-2 MHz), which are considerably less than the GigaHertz frequency range at which many of today's microwave communications and detection systems are designed to operate. The application of a bulk acoustic processor to higher (R.F.) frequencies has not been accomplished because of a number of limitations on the physical and electrical properties of both the launch and the receptor array.
More particularly, as the frequency of the applied signal increases, the extremely close spacing of the physical components and the sheer number of components required (often on the order of several hundred to a thousand) for analyzing a reasonably wide bandwidth (1-10 Ghz) become substantial architecture and performance constraints in the construction and operation of a realizable system. In a large numbered, multi-component transducer array, with hundreds of narrow linewidth transducer elements connected in parallel, the impedance seen by the signal driver may be on the order of thousandths of an ohm, which is considerably less than the 50 ohm impedance of the driver, and thereby severely limits the available bandwidth (which is proportional to the square root-ratio of the driver and source impedances).
One proposal to increase the effective source impedance seen by the driver electronics has been to make a series of physical cuts or scribes in the surface of the bulk material in order to form a series connection of separate portions of the array in series. However, for the extremely narrow linewidths (micron dimensions) required for an array comprised of hundreds of interdigital or finger electrodes, implementing such a proposal is a practically impossible. A second proposal would be to drive the array from a low impedance driver. Again, however, the suggestion is not realistic, as signal amplifiers do not customarily have extremely low output impedances (e.g. in the neighborhood of one to five ohms).